Pressure sensor with housing, sensor element having ceramic components, and support ring mounting sensor element to housing

ABSTRACT

A capacitive pressure sensor for measurement of the pressure of a fluid comprises a sensor element (1) which is made of a ceramic, in particular glass ceramic, material and in the conventional way has interior capacitor plates. The sensor element (1) is plate-shaped and joined to a surrounding support ring (5&#34;). The support ring (5&#34;) is made of metal and is connected to the sensor elemnt (1) by means of a joint (3&#39;) made of ceramics, in particular glass ceramics, at an annular region around an edge line of the sensor element (1). The support ring (5&#34;) can have a shoulder formed by a protrusion (15) in the interior surface thereof, against which shoulder then a region of the sensor element (1) rests and at which the joint (3&#39;) is made.

TECHNICAL FIELD

The present invention relates to pressure transducers or sensors formeasurement of pressures from fluids of various kinds, both gases andliquids, and also to methods of producing pressure transducers orsensors and in particular mounting or attaching a sensor element in atransducer/sensor.

BACKGROUND OF THE INVENTION AND STANDPOINT OF THE ART

In developing the sensor element for a pressure sensor a very largenumber of technical parameters exist which must be considered. Theseparameters are of a thermal, electrical, mechanical or chemical nature.Each such group contains itself a multitude of technical parameters andfurther various properties interact between the groups as applied to asensor element. Chemical characteristics influence surface coatings,which influence thermal and mechanical properties. The density andimpermeability influence possible structural thicknesses what influencesthe mechanical characteristics, etc. A search ol the characteristics ofvarious conceivable materials concludes usually that the followingmaterials can possibly be used: metals of special types such as thealloys "Inconel" and "Hastalloy", semi-conductor materials, for examplesilicon, ceramic materials, for example aluminum oxide.

A very large work has been made in developing different types of sensorelements. In order to use the sensor element it must, however, bemounted in a housing, casing or similar device, to obtain a complete orfinished pressure sensor, which is ready to be used after a simpleconnection to some volume, where a medium is present. However, themounting or attachment method for forming a complete pressure sensor canoften result in the fact that the high precision of the very sensorelement is lost. It depends on the fact that, again, in the mountingprocess problems in regard of the materials used appear which are forexample of the following kind. For metals, their high coefficients ofthermal expansion result in thermal stresses, they are not impermeableto some gases, they are deformable. Semiconductor materials aresensitive to temperature and are easily attacked chemically, what canhave the consequence that a system comprising oil capillaries must beused to conduct the pressure that is to be measured to a pressuresensitive surface. The attachment or securing of parts of ceramicmaterials is often performed, due to the difficulty of working thesematerials and their production at very high temperatures, must often bymeans of O-rings, which do not possess a sufficient chemical resistanceor inertness.

The housing of the pressure sensor is nearly always made of metal andthus, to this metal the very sensor element must be attached or clamped.A suitable ceramic material for use in the construction of sensorelements is glass ceramics, since components or parts of this materialcan be produced and joined/bonded at temperatures which are rather lowin this context. If the sensor element thus is to be bonded directly tometal, it is a critical problem to adapt the coefficient of thermalexpansion of the metal to which the sensor element is to be mounted orclamped, further of the material in the very sensor element and also ofthe bonding or joining material used.

Sensor elements for pressure sensors based on ceramic materials andconstructed as dilatation sensors or capacitive sensors can comprisevarious ceramic materials. Then, often ceramics based on aluminum oxideis used but also glass ceramics is used. In the production of sensorelements based on aluminum oxide the various surface coating processesand the procedures for burning/fusioning the various elements includedin a sensor element to each other must be performed at a significantlyhigher temperature than for glass ceramics. To design, for sensorelements based on aluminum oxide, a sequential order for all theseprocesses comprising surface coating procedures of various kinds andburning/fusion processes, which does not destroy the result of earliercoating processes and other processes in the sequence of proceduralsteps in the production, must be considered very difficult. Further, toadapt for such sensor elements physical parameters, for examplecoefficients of thermal linear expansion, over a wide temperature rangeis also very difficult. Therefrom it is obtained as a naturalconsequence that the high temperatures used imply that mechanicalstresses will exist in a finished sensor element, what has naturallyvarious resulting, difficult effects when using the sensor element in apressure sensor. A rigid securing or attachment of a sensor elementconstructed of ceramic materials to a metal part, for example a ring ofstainless steel, can thus in most cases not be performed owing to thebuilt-in mechanical stresses obtained from the thermotechnicalconditions during the production process of the sensor element.

A prior alternative is to clamp a sensor element, which as conventionalhas the shape of e.g. a flat round plate or chip, to an O-ring, so thatit is pressed against one of the flat surfaces of the sensor element ata region adjacent to the edge of this surface. The clamping force can beproduced by a threaded ring acting on the opposite side of the sensorelement and at a region adjacent the edge of this surface. Anotherpreviously known alternative is that a force from a fluid, the pressureof which is to be measured, is transferred from a primary measuringdiaphragm through an auxiliary fluid to a surface of the sensor element.The clamping of the sensor element can in the latter case beaccomplished in a simpler way, owing to the fact that thecharacteristics of the auxiliary fluid, e.g. silicon oil, which is usedfor transferring the pressure to be measured, are known. Such methodscan however only achieve limited performance due to mechanicalelasticity and mechanical instabilities in an O-ring or a silicon oil,respectively, so that a large accuracy and a rapid sensor responsecannot most often be obtained.

A rigid securing/mounting of the sensor element is according to thediscussion above necessary in order to achieve precision sensors formeasurements in for instance industrial areas, where low pressures areused, such as the semi-conductor industry, but also for pressure sensorsintended for measurement of ordinary pressures a rigidattachment/mounting of the sensor element produces distinctly superiorcharacteristics.

In a prior sensor, see the U.S. Pat. No. 5,249,469, which can be madehaving different dimensions for measuring pressures of differentmagnitudes, both vacuum pressures and atmospherical pressures, thesensor element is almost completely arranged within a volume filled witha fluid the pressure of which is to be measured. The sensor element isattached to a sensor house by means of two slender tubes having a verysmall cross-sectional area of the material of the tubes. In the use ofthe pressure sensor for measurement of low pressures, for measurement ofa vacuum, the whole measurement element is thermotechnically isolatedfrom the surroundings and is influenced little by changes in the ambienttemperature. The heat transport to and from the sensor by means ofconvection and radiation is small and the heat transport occurssubstantially as conduction of heat through the slender tubes.

At rapid temperature changes, in e.g. the use at a low pressure, in arapid pumping down to a vacuum, in a rapid inlet of fluid, the gas, e.g.air, the pressure of which is to be measured, will expand or contract.It results in a small cooling or heating respectively of the gas withinthe measurement volume, with which the sensor element is in contact, andthence also of the sensor element itself.

This phenomenon is in particular embarrassing for a rapid pumping to alow pressure, since the sensor element then, comprising a mountaccording to the discussion above comprising slender tubes, isthermotechnically well isolated both from the housing to which it isattached and which in many cases rather rapidly will adopt the newtemperature due to the fact that it comprises large surfaces and is madeof metal, and from the gas itself. The temperature equalization betweenthe sensor element and the surroundings occurs in this case very slowlythrough the slender tubes and the zero position of the sensor is thendisplaced or offset during a rather long time. The return process to thestate existing before such a change can in the worst cases comprise atime of up to the magnitude of order of hours, which naturally cannot beaccepted. In FIGS. 1a-1c time diagrams are illustrated showing the zeroposition of the output signal for different heat conduction cases inrelation to the surroundings. In the normal case or in the casecomprising a good heat conduction according to FIG. 1a the zero levelthus returns to its normal value after a limited time period. In othercases for which a good thermotechnical isolation is provided between thesensor element and the material in the surroundings, the zero leveldependence of time can look as is illustrated by the curves of FIGS. 1band 1c, comprising the long period mentioned above for a return to thezero level, in FIG. 1b comprising a positive zero point offset and inFIG. 1c a negative zero point offset during a rather long time. Thedifferent curve shapes in FIGS. 1b and 1c depend primarily on theprocesses used in the production of the sensor element itself, i.e. theinterior characteristics thereof. It is possible, in principle, tocompensate for the rapid changes of the zero level in an electronic waybut practically it is combined with large difficulties, since the sensorelement itself is in this construction not easily available forarranging temperature sensors such as thermistors.

A sensor element suspended in two slender tubes is generally not easilyproduced. However, it can be used where a high cost of the pressuresensor can be accepted and for small volumes, for instance in theproduction of semi-conductor elements. For other industrial sensorswhere large numbers of sensors are required but not as extremeperformance, this type of attachment is impossible. Industrial sensorsshould also preferably be able to be operated for measurement ofpressures in different media, both gases and liquids. For liquids themounting by means of two slender tubes does not work.

Another complication in an attachment by means of slender tubes is thewelding thereof to the housing for the sensor element surrounding it.During this welding step, through the slender tubes, mechanical stressesin the very sensor element that is constructed substantially of ceramicmaterial can be very easily introduced, due to the heat generated by thewelding flame used in the welding procedure. The mechanical stressesgenerated in this welding process to the lid or the housing of thesensor element depend on a lot of factors, the diameters of the tubes,the welding velocity, the cooling method, the degree in which it ispossible to simultaneously make the two welds, etc. The final result isthat each sensor mounted in a housing will obtain individualcharacteristics which differ rather much from each other. Thereby theproblem of the accuracy of the sensor is transferred to a finalcalibration of the sensor itself, where these various imperfections mustbe acted on or treated individually and be compensated in differentcomplicated ways.

An alternative to an attachment by means of two slender tubes is bymeans of one single centrally mounted tube, to which a housing portionor annular portion associated with or connected to the sensor element ofa ceramic material is welded.

For an attachment by means of two slender tubes the whole sensor elementis exposed to the pressure to be measured what is an advantage inparticular when measuring pressures above the atmospherical pressure orabove the ambient pressure. All the ceramic parts are then loaded orstressed by the same compression forces what the incorporated ceramicmaterials can stand very well. This advantage for measurements above theatmospherical pressure is not as pronounced for measurements of lowerpressures or for measurements of a vacuum.

For an attachment comprising a single, centrally welded tube, throughwhich the fluid enters the pressure of which is to be measured and whichacts on only one surface of a sensor element, worse characteristics areobtained in the corresponding way for measurements of pressures abovethe ambient pressure. Since only one surface of the pressure element isloaded, the element can be "blown up" approximately in the same way as aballoon. This effect does naturally not exist for pressure sensorsintended for measurements of low pressures and in that case thisattachment method works satisfactorily. Such an attachment by means of acentral tube welded to a housing portion in the shape of a circularplate of metal having on one side an annular projection or bead at thecircumference and having a central aperture, can at present stand anoverpressure of about 10-15 bar. Above this pressure a risk of burstingexists which will be manifest around or at the joints between theceramics parts of which the sensor element is constructed, or by thefact that alternatively the diaphragm in the sensor element breaks. Thesecurity factor against rupture will thereby be low. Owing to therequired processing steps at high temperatures further a distribution ofpressures is obtained for which a rupture occurs, what in additionresults in an unacceptable insecurity in this constructional method.

From the European Patent Application EP-A2 0 549 229 a pressuretransducer is previously known comprising a sensor element having aceramics house 38 and a diaphragm 36 arranged thereon which is made ofmetal (Inconel). Between an exterior stable support ring 42 anintermediate ring 86 is provided, which is welded at its one edgesurface to the stable support ring 42. The other edge surface isconnected at a shoulder or step and by means of glass joint to theceramics house 38, at an angular projection thereon. The intermediatering 86 is made of metal ("Inconel") having a coefficient of thermalexpansion adapted to the material in the housing 38 and has a narrow webbetween the surfaces where the ring 86 is attached to the exteriorsupport ring 42 and to the ceramic; housing 38. This constructionreduces the transfer of the mechanical stresses which can arise attemperature changes due to different thermal expansion coefficients ofthe support ring 42 and the intermediate ring 86. In order to furtherreduce the influence of these stresses the intermediate ring is slotted,see item 98 of FIG. 4. However, such a construction will make themounting of the sensor element less definite, as considered totally, andreduces the precision of the finished pressure sensor. The attachment toan annular projection on the sensor element reduces the thermal transferand the projection can also break rather easily. Further, the fluid thepressure of which is to be measured and which is present at the exteriorsurface of the diaphragm 36, will also act on the whole sensor element,i.e. also on the rear side of the ceramics housing 38, what can be adisadvantage in certain cases, such as for measurement of pressures ofliquids, in the case where a cleaning of the measurement volume may berequired.

DESCRIPTION OF THE INVENTION

It is an object of the invention to provide a pressure sensor having arigid mounting of a ceramic sensor element which can be achieved withoutincurring any substantial risk that the sensor element will break.

It is a further object of the invention to provide a pressure sensorhaving a fixed or rigid securing or attachment of the sensor elementallowing measurements of pressures both in gases and liquids.

It is another object of the invention to provide a pressure sensorcomprising an attachment of the sensor element which satisfies hygienicrequirements cr generally the requirement that it will be possible toget rid of rests of earlier measurement media in and at the portion ofthe sensor which is exposed to a medium the pressure of which is to bemeasured.

It is a further object of the invention to provide a pressure sensor forwhich no essential inherent or incorporated mechanical stresses areintroduced in a sensor element when attaching or mounting the sensorelement.

It is another object of the invention to provide a pressure sensorcomprising a mounting of a sensor element, the mounting including nochemically unstable materials.

It is a further object of the invention to provide a pressure sensorhaving a high repeatability for measurements.

It is a further object of the invention to provide a pressure sensorhaving a good long time stability and presenting small changes of itscharacteristics at temperature changes.

It is a further object of the invention to provide a pressure sensorwhich is available for mounting auxiliary sensors, such as temperaturesensors, on a pressure sensor element.

It is another object of the invention to provide methods for productionof pressure sensors which have a high precision or accuracy.

It is a further object of the invention to provide methods of mountingsensor elements in pressure sensors in order to produce pressure sensorshaving a high accuracy and in particular of connecting a ceramicmaterial to another material by means of an impermeable joint.

The objects mentioned above are achieved by the invention the detailedcharacteristics of which appear from the appended claims.

A sensor element built of parts based on glass ceramics is by means cf ajoint of a glass material attached to an intermediate part of stainlesssteel and hereby a rigid attachment of the sensor element is obtained.The intermediate part of stainless steel is then in turn welded in asensor housing or casing, possibly through another stainless part.

For this basic construction there will in the pressure sensor be nochemically unstable materials which are in contact with fluid within ameasurement volume in which the pressure is to be measured. It is inparticular important in the use for low pressures, for a vacuum.However, it is considerably more important, as has been indicated above,that a rigid attachment of the sensor element distinctly improves anumber of technical characteristics, e.g. repeatability generally for anelectrically detected output quantity, repeatability within themeasurement range and also, which is very important, a repeatability foroverpressure loads. Hereby hysteresis in the pressure sensor can beeliminated to such a high extent that it cannot be demonstrated orproved. The long time stability is enhanced and changes owing totemperature changes are reduced, compare e.g. a sensor element clampedby means of an O-ring, the material properties of which at differenttemperatures strongly influence the result of the measurement for such aclamped sensor.

The material in the stainless part to which the very sensor element iswelded, is selected to have adapted properties, in particular in regardof thermal expansion at various temperatures, so that steel having avery particular composition must often be used. Further, this metal partmust be heat-treated in a particular way in order that the attachmentbetween the sensor element of ceramics to the material of the metal partwill work without producing, neither at the production or in the use ofthe pressure sensor, ruptures or breaks in the sensor element.

The sensor element itself is constructed of ceramic plates or chips,processed in different ways, which are joined or bonded to each other bymeans of joints of a glass material. At the exterior circumference ofthe sensor element, in particular at only part of the exterior edge, aglass joint is obtained between the ceramic material of the sensorelement and a surrounding exterior stainless steel ring. The stainlesssteel ring is designed to have a shoulder or step, which rests againstthe exterior marginal portion of one of the large surfaces of theessentially plate-shaped sensor element. The sensor element itself isalso, in a preferred way, designed to have a rather small diameter,practically such a so small diameter as is allowed by the actualconditions, for instance of the magnitude of order of 10-20 mm andpreferably within the range of 10-15 mm. In particular cases, where onlythe very diaphragm of the sensor element is allowed to be exposed to thefluid the pressure of which is to be measured, such as for liquidswithin the food manufacturing industry, the glass joint must extend overall of the cylindrical exterior edge of the sensor element so that noslot or recess is formed between the exterior circumferential edge ofthe sensor element and the interior envelope surface of the supportring. In this case the glass joint will have a rather long extension inthe cross direction of the sensor element and therefore the paste orcompound, which contains finely divided glass particles and which iscoated in producing the glass joint, before the heating for melting orfusioning the glass particles, can be coated in a suitable dotted orchannelled pattern having regular inner portions where no glass jointpaste is deposited. Thereby those various gases which are generated inthe heating for obtaining the very joint, are allowed to be let out fromthe joint during the heating process, and still a completely tight jointis obtained. A narrow slot at the circumference of the sensor elementcan also possibly be avoided by a suitable design of the inner portionof the stainless ring, for instance in such a way that it will be morenarrow or tapering in the direction towards its center axis.

For sensors intended for low pressures comprising thin measurementdiaphragms it can be advantageous if on this diaphragm side an extrapart is arranged such as a counterplate or counterring. A portion,projecting axially and located at the edge of the counterplate or a flatannular surface of the counterring, respectively, is joined or bonded tothe thin diaphragm by means of a glass joint. A counterplate is in thatcase provided with through holes in order that the pressure from themedium the pressure of which is to be measured will reach the verymeasurement diaphragm, which resides below the counterplate. Such anarrangement comprising a counterpart results in the effect that themeasurement diaphragm will be more uniformly attached at its portionlocated at the circumference. The counterplate or counterring will thusstabilize the thin diaphragm.

The glass joint between the thin movable diaphragm and the house part ofthe sensor element, which is a generally considerably thicker ceramicplate, is very thin and thus forms the part, which is the partdistinguishing the movable diaphragm and the sensor housing part frombeing a homogeneous mechanical body. Temperature gradients over thejoint causes mechanical stresses which result in a displacement of themovable diaphragm and thereby an incorrectness of the output signalwhich is detected at the displacement of the diaphragm when influencedby the pressure from the measurement medium. It is particularlyobservable for thin diaphragms for measuring low pressures. Temperaturegradients are obtained during the various heating steps in themanufacture and in the use of a pressure sensor also from thesurroundings or from the medium the pressure of which is to be measured.These two cases can be treated in different ways.

The velocity with which a temperature change of the sensor element isobtained, has a large importance to the temperature drift of the sensorelement. In the best case a temperature change occurs slowly so thatcorrespondingly also a slow change of the zero level of the outputsignal occurs. This type of incorrectness in the output signal can behandled. The glass ceramic material which is preferred for the ceramicparts of the sensor element, has in addition the very positive propertythat it provides a linear relation between temperature change and errorsignal. Such an effect can be simply compensated electronically.

This case comprising slow temperature changes is illustrated by thediagram of FIG. 2a, where the output signal for a constant pressure ofthe measurement medium is plotted as function of time. For a rapidtemperature change an output signal response is obtained of the typeillustrated by the curves of FIGS. 2b and 2c. The final level after thetemperature changes according to FIGS. 2b and 2c is the same as for thecase illustrated by the curve of FIG. 2a and it can be electronicallycompensated. However, the intermediate region having a more or less highvalue of the derivative of the curve cannot be compensated in this way.

Rapid temperature gradients can be influenced by "buffering" the sensorelement, i.e. hiding it from temperature changes in the surroundings. Itcan be accomplished in a number of different ways, for instance by meansof an extremely heavy heat isolation, for instance corresponding to afilling of mineral wool having a thickness of 2 m, but such measuresmust always be balanced against technical aspects of manufacture and thebenefit which it can imply to the user. If the sensor element isbuffered thermotechnically in this way against the surroundings, alsofor rapid temperature changes a signal response is obtained of the typeillustrated in FIG. 2a, which can be electronically compensated so thatthe resulting output signal will have a smooth or even behaviour.

If a deviation of the output signal for a temperature change cannot becompensated, it should in the best case have a shape according to thecurve illustrated in the diagram of FIG. 2b, comprising a shortspike-like pulse at the very temperature change.

Temperature changes in the medium the pressure of which is to bemeasured should preferably take place between different constant levels.For gaseous media normally no large problems are obtained owing to thesmall heat capacity of gases. Temperature changes in the medium shouldpreferably, by a suitable design of the sensor element, influence itrapidly and the temperature should be evened out or equalized in thesensor element as quickly as possible. The preferred cases for thetemperature changes are, as has already been indicated, the curvesillustrated in FIGS. 2a and 2d.

A sensor element of this kind having such an attachment by means of aglass joint to a surrounding metal ring has the following advantages:

It is possible to use the sensor element for both gases and liquids. Thesensor element can be used as an absolute pressure sensor and as adifferential sensor, e.g. in relation to the atmospherical pressure.

The sensor element is rigidly attached by means of a glass joint and aweld in a sensor housing of stainless steel.

In a preferred case the diaphragm is rigidly attached or secured fromtwo directions comprising a counterpart such as a plate or a ring.

The sensor element is available from the outside for electroniccompensation and temperature sensors such as thermistors which can bearranged on the element.

The element can accommodate, in the shape of compression stresses, theforces from the measurement, i.e. the pressure from the medium thepressure of which is to be measured results only in mechanicalcompression stresses in the sensor element.

Only a "naked" diaphragm is exposed to the measurement medium.

The construction comprising an exterior attachment ring to which thevery sensor element is joined or attached, stabilizes the ceramic sensorelement itself.

Temperature gradients owing to temperature changes in the surroundingsare minimized.

An adaption occurs rapidly to the temperature in the medium the pressureof which is to be measured.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail and with reference tonon-limiting embodiments and with reference to the accompanying drawingsin which:

FIGS. 1a, 1b and 1c illustrate the displacement of the zero level of theoutput signal as a function of time for a sensor which is unloaded andwell heat-isolated from the surroundings, when a temperature changetakes place in the medium the pressure of which is measured,

FIGS. 2a, 2b, 2c and 2d illustrate the displacement of the output signalas a function of time for a change of temperature of the medium thepressure of which is to be measured, for a better isolated sensor andfor a predetermined pressure in the medium,

FIG. 3 is a section of a pressure sensor for overpressures or forpressures which do not deviate too much from the atmospherical pressure,

FIGS. 4 and 5 illustrate the attachment of a sensor element formeasurement of overpressures and for measurement of low pressures,respectively,

FIG. 6 is a section of a sensor intended for measurement of pressures,where particular requirements are set,

FIGS. 7a, 7b and 7c are sections of the attachment of a sensor elementcomprising a uniform attachment of a diaphragm in the sensor element.

DETAILED DESCRIPTION

Glass ceramic material has typically a rather linear thermal expansionfor temperatures between ambient temperature and about 500° C. At about475-500° C. the thermal expansion coefficient increases to a largervalue. Such a material is to be joined to a metal by means of a glassjoint. The joint region will thereby be constructed of the threematerials glass ceramics, glass joint and metal. In producing the jointa complete assembly of glass ceramics/glass/metal is heated until thejoint glass melts. During the cooling process then the glass in thejoint will solidify. In a further cooling the glass will achieve its"strain point" which is the highest temperature from which the glass canbe cooled without building permanent mechanical stresses into the glass.In order to achieve a condition which is as free from stresses as ispossible, in the joint region, thus, at temperatures lower than thistemperature the thermal expansion and contraction characteristics of theincluded materials must be as similar as possible.

To achieve a glass joint, in the particular case a joint between theglass ceramic material and the metal, a paste or compound is usedcontaining a binding agent, preferably an organic binding agent togetherwith a solvent therefor and glass particles. The binding agent keeps theglass particle together so that the paste can be deposited in a selectedthickness and in a suitable pattern on one of the surfaces which is tobe joined, usually a surface on the glass ceramics material. Suitabledeposition methods can be screen printing and pad/brush printing. Theglass material in the particles must be chosen so that it has thermallinear expansion characteristics adapted to that of the glass ceramicsaccording to the above and it should be such that it melts and can flowcompletely and adhere within a temperature range of about 450 to 530°C., the so called "sealing region", since the curve of the thermallinear expansion of the glass ceramics changes its slope at about 475°C. and the highest temperatures used should not be too much above thistemperature. After deposition of the glass paste normally the followingsteps are performed:

Drying at 100-150° C.

A "burn-off" period at 325-375° C.

A "prefusion"/sintering at 490-500° C.

In the drying process the deposited layer becomes hard so that it canresist handling. The "burn-off" stage is very important. During thisstep organic solvents and binding agents are removed. If it is not madein an adequate way the joint will not be impermeable and notsufficiently mechanically strong. Then, when the joint material issufficiently degassed, the temperature is elevated to a temperaturewithin the "sealing region", for instance at a temperature a littlelower than the upper limit of this range, for execution of the step"prefusion"/sintering. Then the glass particles are melted together orfusioned, at least partly at their surfaces, without flowing completely.After that the joint material is allowed to cool. The joint material hasnow become quite hard and has rather well been rid of gases andmaterials developing gases at temperatures within the "sealing region"but however, some rests still remain.

A joint layer can be given a well-defined thickness after execution ofthe steps above, if suitable deposition methods are used, so that thelayer will after the step of "prefusion"/sintering for example have athickness of 25 μm. The joint layer can after this step be mechanicallyworked, e.g. polished, so that it becomes still thinner, which can beadvantageous in many cases. It can be suitable to provide such apolishing so that the sintered layer will have a thickness of about 5 μmand thereby the finished joint will have a thickness less than thisvalue.

At last the surfaces which are to be joined to each other, are placedagainst each other with the joint layer located therebetween. All of thejoint region is heated to a temperature within the higher portion of the"sealing region", for instance comprising a peak temperature of about530° C., where the glass material will be completely liquid and fillsthe joint. Then also, always a compression force is applied over thejoint region.

In order to facilitate, in the final degassing during the very joiningprocess, when the metal material is applied against the glass ceramicsmaterial having the joint layer located therebetween and when atemperature within the uppermost portion of the "sealing region" isused, the joint material is deposited advantageously as a dotted orchannelled pattern, in which interspaces and/or channels are providedbetween regions of glass paste. Typical patterns are shown in FIGS. 3aand 3b, where the former figure shows a pattern in the shape of obliquesquares, the joint material being located within areas having a rhombicor parallel-epipedic shape with the points located adjacent to eachother, so that regions having the corresponding shape are providedtherebetween. In the latter form the deposited glass paste areas havethe shape of hexagons having channels located therebetween. The size ofthe isles and the dimensions of the interspaces or channels locatedtherebetween are chosen so that the desired degassing effect can beachieved. The isles can typically have a largest measure or diameter of0.1-0.5 mm comprising interspaces or channels having widths of the samemagnitude of order.

A further advantage found in the deposition of joint materials in anon-contiguous layer but as separate small areas, evenly distributedover the joint area can be that the finally finished joint, after thefinal melting or fusioning will be thinner than the earlier depositedlayer itself of glass paste or even thinner than a layer, which possiblyis worked to a small thickness, after the sintering step. Thedistribution of the isles in the deposited glass paste layer must thenbe selected also considering the desired final thickness of the glassjoint.

In the final joining in a heating to a temperature within the higherpart of the "sealing region", according to what has been said above acompression force is applied over the joint region, e.g. by loading theceramic material or the metal material with the weight of suitableweights. Remaining gas rests can then exit due to the dotted patterning,as illustrated by FIGS. 4a and 4b. In FIG. 4a a section is illustratedof a joint area having joint material 401 deposited in separate,adjacent isles having a thickness of 5 μm after the sintering step. Thejoint material is located between the two parts 403 and 405 which are tobe joined. The joint area is heated to a suitable temperature and thengas residuals can exit, see the arrows 407. At the same time the parts403 and 405 are pressed towards each other for achieving a compressionof the joint layer, which is illustrated by the arrows 409. Owing to thedegassing process, the compression force and the high temperature theglass material in the isles 401 can flow completely in order to form acompletely filled and impermeable joint 411, as is illustrated in thecorresponding section of FIG. 4b. Then naturally, the thickness of thejoint decreases considerably.

The discussion above means, that in joining to a metal metal materialhaving a coefficient of thermal linear expansion adapted to that of theglass and the glass ceramics, must be used, having the best possibleagreement within the temperature range mentioned from somewhat below500° C. to ambient temperature. Suitable metal materials should furtherhave a good chemical resistance and they must also have furthercharacteristics, such as that it must be possible to weld the materials,they must be able to be worked mechanically, etc. Among the stainlesssteels iron-nickel alloys can be found which at least approximately havethe desired properties of thermal linear expansion and which alsosatisfy the other requirements reasonably.

In FIG. 5 thus a diagram is illustrated of the thermal linear expansionas a function of temperature for a typical glass ceramic material. Thecurve has a linear region between a temperature below 0° C. and somewhatbelow 500° C. and it is within this region where the coefficient ofthermal linear expansion of the two materials must be adapted to eachother. At 501 and 503 further, corresponding curves are illustrated fortwo conceivable NiFe-alloys, the former having a somewhat higher thermallinear expansion and the latter having a somewhat lower thermal linearexpansion than the glass ceramics. Otherwise, the curves are wellparallel within the considered temperature region. It is also possibleto produce alloys having a still better adaption than what appears fromthe diagram but it can be suitable, depending on the design case, to usemetal materials having somewhat deviating thermal linear expansions, sothat for example the ceramic material, in the heating process, is alwaysrather exposed to compression forces than tension forces over a jointregion.

In FIG. 6 a section is illustrated of a capacitive pressure sensorintended for overpressures or moderate underpressures. A sensor element1 comprising capacitor plates, not shown, such as an electricallyconducting plate on an interior surface of a diaphragm, not shown, andan opposite conducting plate on an interior surface of a thicker housepart, not shown, has the shape of an essentially circular cylindricalplate-shaped unit 1 made of different parts, not shown, which are basedon glass ceramic materials, and is joined by means of glass joints 3against a support ring 5. The support ring 5 of stainless steel havingadapted characteristics according to what has been said above and havinga particular composition is welded by means of an annular weld 7 to amounting ring 9, which surrounds radially the support ring 5. Themounting ring 9 is connected by means of another annular weld 10 to asensor housing 11, e.g. as is illustrated in the figure at an exterioredge of the mounting ring 9. The sensor housing 11 has a main partshaped as a plate having both an annular bead or platform projectingaxially for connection to the mounting ring 9 and a centrally locatedaperture 13, through which the medium enters the pressure of which is tobe measured. At the aperture 13 a suitable thread may be arranged or anipple or sleeve provided with interior or exterior thread may beattached, not shown in the figure, for attachment of the whole pressuresensor to a fluid line, not shown, for the medium the pressure of whichthe pressure sensor is intended to measure.

In FIG. 7 essential details of the pressure sensor in FIG. 6 are shown,i.e. the sensor element 1 and the interior and exterior rings 5 and 9,also in this case shown sectionally, in a somewhat modified embodiment.The support ring 5 thus has an interior shoulder at its one side, sothat an annular, inwardly projecting protrusion 15 is formed at one sideof the support ring 5. Against this protrusion 15 rests, through ajoint, a side surface of the ceramic sensor element 1. The sensorelement 1 is also joined to the support ring 5 at a portion of itsexterior, essentially cylindrical envelope surface along an essentiallycylindrical, annular surface, which connects to the engagement surfaceagainst the shoulder formed by the protrusion 15. Actually, theexterior, essentially cylindrical surface of the sensor element is veryweakly conical, having a cone angle of for example about 85-88° in orderthat, in the joining process of the support ring to the sensor element,it will be possible to apply a compression force over the exterior jointregion. The essentially cylindrical or weakly conical joint surfacehowever, does not extend over all of the envelope surface of the sensorelement 1 but only over a part thereof. The total width of the joint 3,which is formed of glass material at the joint surfaces between thesensor element and the support ring 5, is however always of the samemagnitude of order as the thickness of the sensor element in order toobtain a good heat transfer and a secure attachment. By arranging such awide joint also a satisfactory mechanical strength is accomplished andfurther a completely impermeable joint, so that the medium cannot reachthe rear side of the sensor element 1.

In order to produce the short exterior annular joint surface between thesensor element 1 and the support ring 5 still another shoulder or step,shown at 15, is provided in the interior surface of the support ring 5.

The mounting ring 9 can be made of a conventional stainless steel andthe welding joint 7 between the mounting ring 9 and the support ring 5is in this case arranged on the side where the pressure force acts foroverpressures, i.e. at the same side where the diaphragm, notillustrated in this figure, is located in the sensor element 1. Further,the engagement surface between the support ring 5 and the mounting ring9 is made frusto-conical or tapering away from the side of the sensorelement 1, where the pressure of the measurement medium acts, also foradopting or accommodating pressure forces in the use of the pressuresensor for overpressures.

Electrically conducting wires 19, see FIG. 6, pass through holes orrecesses, not shown, in the protrusion 15 which projects radiallyinwards, for connection of electrical conductors, not shown, inside thesensor element 1 to electronic circuits on a circuit board 21, thatthrough some suitable support device such as a cylindrical ring 23 of asuitable material is attached to the mounting ring 9.

The embodiment shown schematically in FIG. 6 of the mounting of thesensor element 1 differs somewhat from that illustrated in FIG. 7. Thethickness of the support ring 5 as seen in the axial direction can inthe embodiment according to FIG. 6 be essentially equal to the thicknessof the sensor element 1, but in the embodiment according to FIG. 7 thethickness of the support ring 5 is larger than that of the sensorelement, more particularly in the latter case so that the thickness ofthe support ring 5 corresponds to the sum of the thickness of the sensorelement 1 and the axial width of the protrusion 15. In this case thus,the width of the interior envelope surface of the support ring 5, wherethe shoulder 17 is located, is essentially equal to the thickness of thesensor element. In the embodiment according to FIG. 6 the protrusionagainst which the sensor element 1 rests is designed to have aconsiderably larger thickness, as seen in an axial direction, and in thecorresponding way the width as seen in the axial direction of theinterior envelope surface of the support ring 5, which faces theexterior circumferential edge of the sensor element 1, is smaller andcorresponds to the axial length between the shoulder 17 and theengagement surface of the supporting protrusion 15 in the embodimentillustrated in FIG. 7.

In FIG. 8 are shown, as seen sectionally, the same parts as in FIG. 7designed for measurement of low pressures, i.e. for a vacuum sensor. Theembodiment of FIG. 8 differs from that of FIG. 7 by the fact that thevarious force receiving surfaces are located for accommodating forcesdirected from the opposite direction, for FIG. 7 forces originating froma point at the lower portion of the paper, as this figure is drawn, andfor FIG. 8 for forces acting from above, from the top portion of thedrawing sheet. The inwardly projecting, annular protrusion 15' on thesupport ring 5' is thus, in the embodiment according to FIG. 5, locatedon the side of the pressure sensor where the medium is intended to belocated, the pressure of which is to be measured. Also the glass joint 3between the sensor element 1 and the mounting ring 5 is located on thisside. The shoulder 17' on the interior cylindrical surface in themounting ring 9 will here be directed oppositely compared to theshoulder 17 according to FIG. 7. The conical surface of separationbetween the interior support ring 5' and the exterior mounting ring 9'has here the opposite conicity, i.e. the surface of separation will bemore narrow in the direction towards the side of the sensor elementwhere the medium is intended to be present the pressure of which is tobe detected.

For some applications where special requirements exist, such as withinthe foodstuff manufacturing industry, for the kind of sensor accordingto FIG. 7 the slot or recess 25 cannot be accepted, which is formed bythe arrangement of the shoulder 17 for reduction of the axial length ofthe glass joint 3. In such cases, for measurement of overpressures orpressures about the atmospherical pressure, the glass joint 3' between:he sensor element 1 and the support ring located directly outside itmust be made over the whole width of the sensor element 1 in order toobtain a sufficient strength. An example thereof is shown in FIG. 9where a section of such a sensor is schematically depicted. The supportring 5" is here thicker in the axial direction, perpendicularly to thelarge surfaces of the sensor element 1, than in the embodiment accordingto FIG. 6, and like the embodiments illustrated in FIGS. 7 and 8 havinga somewhat larger thickness, as seen in an axial direction, than thesensor element 1. The glass joint 3' is here made in the particularmanner, as has been described above, by depositing the material which isto form the glass joint, in a dotted manner or having channels, so thatempty regions are formed between regions comprising material. The areas,where no material has been deposited, must be able to form channels upto a free edge of the joint 3'. The dotted or patterned deposition,however, must not necessarily be made over all of the joint area butmainly in the inner portion thereof. In this way formations of largerair or gas enclosures are avoided, i.e. formation of bubbles, etc. Whenheating this material containing finely divided glass material in asuitable cohesive or binding material, the glass material flows, at asuitable heating, all over of the joint region. Then most of thecohesive material also passes away or exits through the channels whichare formed between the ridges in the pattern.

In the case comprising thin diaphragms which are used for measurement ofvery low pressures a further reinforcement can be arranged for thediaphragm by arranging fact that the reinforcing element is attached tothe marginal region of the thin diaphragm. This is schematically shownin the sections of FIGS. 10a, 10b and 10c. It is seen here that thesensor element 1 in the conventional way comprises a thick plate orhouse part 27 and a thin diaphragm 29 arranged at a small distance fromthe house part 27. At the opposite side of the diaphragm 29, which isturned away from the housing part 27 and which is intended to face themedium the pressure of which is to be measured thus a reinforcementelement is attached having the shape of a plate 31 that comprises anannular projection and through-holes in the interior region of the plateaccording to FIG. 10a. The counterplate 31 can be of a suitablematerial, e.g. of glass ceramic materials like the other parts of thesensor element 1 itself. The reinforcement element can also beconstituted by an annular element 31' according to FIG. 10b. For thetype of sensors according to FIG. 9 having no axial slot at thecircumference of the sensor element 1 a counterplate of the typeaccording tc FIG. 10a can be used, as is illustrated in FIG. 10c. Thesensor element 1' will hereby be constituted by a thicker assembly whichresults in that also the height in the axial direction of thesurrounding support ring 5' in this case must be increasedcorrespondingly so that the interior surface of the support ring 5'encloses completely the cylindrical envelope surface of the assembledsensor element 1' including the counterplate 31.

The manufacture of a pressure sensor according to FIG. 9 will now besummarized. The housing 11 and the exterior mounting ring 9 are supposedto be already provided and be of suitable stainless steels. The sensorelement 1 is first assembled more or less finally of parts made ofsuitable ceramic materials having electrically conducting layers thereonor therebetween. Its exterior edge surface is given a weaklyfrusto-conical shape according to what has been described above. Thesupport ring 5" is also made of metal having an interior surface adaptedto the edge surface of the sensor element and then also having acorrespondingly weakly frusto-conical shape. A layer of joint materialis deposited on the edge surface of the sensor element 1, over the wholeengagement surface of the support ring to the sensor element, when it isplaced around the element. Advantageously, the joint material is dottedor patterned according to what has been described above. The sensorelement is placed inside the support ring surrounded thereby and thesupport ring is connected to the sensor element by heating and thenapplying suitable forces on the support ring 5" and the sensor element1, so that the joint material is exposed to a compression force and thematerial in the joint layer flows and forms a full or contiguous joint.The support ring 5' is attached to the mounting ring 9 by means ofwelding.

In the production of the support ring it is in particular observed thatthe support ring is produced of a metal material having an adaptedcoefficient of thermal linear expansion, i.e. that it has a coefficientof thermal linear expansion which is essentially equal to thecoefficient of thermal linear expansion of the ceramic material of whichthe sensor element is produced. Further, the support ring is produced asa whole ring having no slots, apertures or windows or similar devices.

What is claimed is:
 1. A pressure sensor for measuring the pressure of afluid, comprising:a housing, positionable in contact with a volumecontaining the fluid, a sensor element comprising components made of aceramic material, the sensor element being plate-shaped having athickness and further having two large surfaces and an edge surface, theedge surface having a width and extending around a circumference of thesensor element, the sensor element further enclosing an interior volumewhich changes when a pressure acting on at least part of the sensorelement changes, and the sensor element further comprising electricalconductor paths having electrical characteristics which change when theinterior volume changes, and a support ring made of metal, surroundingthe sensor element and rigidly connecting the sensor element to thehousing, the sensor element being mechanically attached to the housingonly by the support ring, wherein the support ring is directly connectedto the sensor element by a ceramic joint at at least a portion of theedge surface of the sensor element, and wherein the metal of the supportring has a coefficient of thermal linear expansion substantially equalto the coefficient of thermal linear expansion of the ceramic materialin the sensor element.
 2. The pressure sensor of claim 1, wherein thejoint has a width substantially equal to the width of the edge surface.3. The pressure sensor of claim 1, wherein the support ring has athickness taken in a direction perpendicular to the large surfaces ofthe sensor element not less than the thickness of the sensor element andwherein a joint between the support ring and the sensor element extendsover substantially all of the edge surface of the sensor element.
 4. Thepressure sensor of claim 1, wherein the support ring is an impermeablering, so that a fluid cannot pass from one side of the support ring andthe sensor element to another, opposite side of the support ring and thesensor element through the support ring.
 5. The pressure sensor of claim1, wherein the support ring is provided with a shoulder in a surfacefacing a center axis of the support ring, with which shoulder a marginalregion of one of the large surfaces of the sensor element is engaged. 6.A pressure sensor for measuring the pressure of a fluid, comprising:ahousing positionable in contact with a volume containing the fluid, asensor element comprising components made of a ceramic material, thesensor element being plate-shaped having a thickness and further havingtwo large surfaces and an edge surface, the edge surface having a widthand extending around a circumference of the sensor element, the sensorelement further comprising an interior volume which changes when apressure acting on at least part of the sensor element changes, and thesensor element further comprising electrical conductor paths havingelectrical characteristics which change when the interior volumechanges, and a support ring made of metal and surrounding the sensorelement, the support ring connecting the sensor element to the housing,wherein the sensor element has a cylindrical or frustoconical shape, sothat the edge surface constitutes the envelope surface of a cylinder ora frustum of a cone, respectively, and wherein the support ring isdirectly attached to the sensor element by means of a ceramic joint atat least a portion of the edge surface of the sensor element.
 7. Apressure sensor for measuring the pressure of a fluid, comprising:ahousing positionable in contact with a volume containing the fluid, asensor element comprising components made of a ceramic material, thesensor element being plate-shaped having a thickness and further havingtwo large surfaces and an edge surface, the edge surface having a widthand extending around a circumference of the sensor element, the sensorelement further comprising an interior volume which changes when apressure acting on at least part of the sensor element changes, and thesensor element further comprising electrical conductor paths havingelectrical characteristics which change when the interior volumechanges, and a support ring made of metal and surrounding the sensorelement, the support ring connecting the sensor element to the housingand having a ceramic joint to the sensor element, wherein the joint hasa width substantially equal to the width of the edge surface of thesensor element.
 8. A pressure sensor for measuring the pressure of afluid, comprising:a housing, positionable in contact with a volumecontaining the fluid, a sensor element comprising components made of aceramic material, the sensor element being plate-shaped having athickness and further having two large surfaces and an edge surface, theedge surface having a width and extending around a circumference of thesensor element, the sensor element further comprising an interior volumewhich changes when a pressure acting on at least part of the sensorelement changes, and the sensor element further comprising electricalconductor paths having electrical characteristics which change when theinterior volume changes, and a support ring made of metal andsurrounding the sensor element, the support ring connecting the sensorelement to the housing and having a ceramic joint to the sensor element,wherein the thickness of the support ring taken in a directionperpendicular to the large surfaces of the sensor element is not lessthan the thickness of the sensor element, and wherein the ceramic jointbetween the support ring and the sensor element extends oversubstantially all of the edge surface of the sensor element.
 9. Apressure sensor for measuring the pressure of a fluid, comprising:ahousing, positionable in contact with a volume containing the fluid, asensor element comprising components made of a ceramic material, thesensor element being plate-shaped having a thickness and further havingtwo large surfaces and an edge surface, the edge surface having a widthand extending around a circumference of the sensor element, the sensorelement further comprising an interior volume which changes when apressure acting on at least part of the sensor element changes, and thesensor element further comprising electrical conductor paths havingelectrical characteristics which change when the interior volumechanges, and a support ring made of metal and surrounding the sensorelement, the support ring connecting the sensor element to the housingand having a ceramic joint to the sensor element, wherein the supportring is an impermeable ring, so that a fluid cannot pass from one sideof the support ring and the sensor element to another, opposite side ofthe support ring and the sensor element through the support ring. 10.The pressure sensor of claim 6, wherein the joint has a widthsubstantially equal to the width of the edge surface.
 11. The pressuresensor of claim 6, wherein the support ring has a thickness taken in adirection perpendicular to the large surfaces of the sensor element notless than the thickness of the sensor element and wherein the ceramicjoint between the support ring and the sensor element extends oversubstantially all of the edge surface of the sensor element.
 12. Thepressure sensor of claim 6, wherein the support ring is an impermeablering, so that a fluid at one side of the support ring and the sensorelement cannot pass to another, opposite side of the support ring andthe sensor element through the support ring.
 13. The pressure sensor ofclaim 6, wherein the support ring is provided with a shoulder in asurface facing a center axis of the support, with which shoulder amarginal region of one of the large surfaces of the sensor element isengaged.
 14. The pressure sensor of claim 7, wherein the support ring isan impermeable ring, so that a fluid at one side of the support ring andthe sensor element cannot pass to another, opposite side of the supportring and the sensor element through the support ring.
 15. The pressuresensor of claim 7, wherein the support ring is provided with a shoulderin a surface facing a center axis of the support, with which shoulder amarginal region of one of the large surfaces of the sensor element isengaged.